Internet Draft Editor: Peter Gutmann
draft-ietf-smime-password-05.txt University of Auckland
August 26, 2001
Expires February 2002
Password-based Encryption for CMS
Status of this memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
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The list of current Internet-Drafts can be accessed at
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The list of Internet-Draft Shadow Directories can be accessed at
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1. Introduction
This document describes a password-based content encryption mechanism
for CMS. This is implemented as a new RecipientInfo type and is an
extension to the RecipientInfo types currently defined in RFC 2630
[RFC2630].
The format of the messages are described in ASN.1 [ASN1].
The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
interpreted as described in [RFC2119].
1.1 Password-based Content Encryption
CMS currently defined three recipient information types for public- key
key wrapping (KeyTransRecipientInfo), conventional key wrapping
(KEKRecipientInfo), and key agreement (KeyAgreeRecipientInfo). The
recipient information described here adds a fourth type,
PasswordRecipientInfo, which provides for password- based key wrapping.
1.2 RecipientInfo Types
The new recipient information type is an extension to the RecipientInfo
type defined in section 6.2 of CMS, extending the types to:
RecipientInfo ::= CHOICE {
ktri KeyTransRecipientInfo,
kari [1] KeyAgreeRecipientInfo,
kekri [2] KEKRecipientInfo,
pwri [3] PasswordRecipientinfo -- New RecipientInfo type
}
Although the recipient information generation process is described in terms of
a password-based operation (since this will be its most common use), the
transformation employed is a general-purpose key derivation one which allows
any type of keying material to be converted into a key specific to a particular
content-encryption algorithm. Since the most common use for password-based
encryption is to encrypt files which are stored locally (rather than being
transmitted across a network), the term "recipient" is somewhat misleading, but
is used here because the other key transport mechanisms have always been
described in similar terms.
1.2.1 PasswordRecipientInfo Type
Recipient information using a user-supplied password or previously
agreed-upon key is represented in the type PasswordRecipientInfo. Each
instance of PasswordRecipientInfo will transfer the content-encryption
key (CEK) to one or more recipients who have the previously agreed-upon
password or key-encryption key (KEK).
PasswordRecipientInfo ::= SEQUENCE {
version CMSVersion, -- Always set to 0
keyDerivationAlgorithm
[0] KeyDerivationAlgorithmIdentifier OPTIONAL,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
encryptedKey EncryptedKey }
The fields of type PasswordRecipientInfo have the following meanings:
version is the syntax version number. It MUST be 0. Details of the
CMSVersion type are discussed in CMS [RFC2630], section 10.2.5.
keyDerivationAlgorithm identifies the key-derivation algorithm, and
any associated parameters, used to derive the KEK from the user-
supplied password. If this field is absent, the KEK is supplied from
an external source, for example a crypto token such as a smart card.
keyEncryptionAlgorithm identifies the key-encryption algorithm, and
any associated parameters, used to encrypt the CEK with the KEK.
encryptedKey is the result of encrypting the content-encryption key
with the KEK.
1.2.2 Rationale
Password-based key wrapping is a two-stage process, a first stage in
which a user-supplied password is converted into a KEK if required, and
a second stage in which the KEK is used to encrypt a CEK. These two
stages are identified by the two algorithm identifiers. Although the
PKCS #5v2 standard [RFC2898] goes one step further to wrap these up
into a single algorithm identifier, this design is particular to that
standard and may not be applicable for other key wrapping mechanisms.
For this reason the two steps are specified separately.
The current format doesn't provide any means of differentiating between
multiple password recipient info's, which would occur for example if
two passwords are used to encrypt the same data. Unfortunately there
is a lack of existing practice in this area, since typical applications
follow the model of encrypting data such as a file with a single
password obtained from the user. Two possible options would be to use
an OCTET STRING hole or a SEQUENCE OF everything-imaginable OPTIONAL,
however without any clear indication of what's required it's probable
that every implementation will choose to interpret the field
differently, leading to non-interoperability between applications.
Given this incompleteness, an appropriate mechanism would be difficult
(perhaps impossible) to define at this time. If sufficient demand
emerges then this may be addressed in a future version of this
document, for example by adding an optional identification field of an
appropriate form.
2 Supported Algorithms
This section lists the algorithms that must be implemented. Additional
algorithms that should be implemented are also included.
2.1 Key Derivation Algorithms
These algorithms are used to convert the password into a KEK. The key
derivation algorithms are:
KeyDerivationAlgorithmIdentifer ::= AlgorithmIdentifier
Conforming implementations MUST include PBKDF2 [RFC2898]. Appendix B
contains a more precise definition of the allowed algorithm type than
is possible using 1988 ASN.1.
2.2 Key Encryption Algorithms
These algorithms are used to encrypt the CEK using the derived KEK.
The key encryption algorithms are:
KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier
The PasswordRecipientInfo key encryption algorithm identifier is:
id-alg-PWRI-KEK OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 9 }
The AlgorithmIdentifier parameters field for this algorithm contains
the KEK encryption algorithm used with the the key wrap algorithm
specified in section 2.3.
There is no requirement that the CEK algorithm match the KEK encryption
algorithm, although care should be taken to ensure that, if different
algorithms are used, they offer an equivalent level of security (for
example wrapping a Triple-DES key with an RC2/40 key leads to a severe
impedance mismatch in encryption strength).
Conforming implementations MUST implement the id-alg-PWRI-KEK key wrap
algorithm. For the KEK encryption algorithms used by id-alg-PWRI-KEK,
conforming implementations MUST include Triple-DES in CBC mode and MAY
include other algorithms such as AES, CAST-128, RC5, IDEA, Skipjack,
Blowfish, and encryption modes as required. Implementations SHOULD NOT
include any KSG (keystream generator) ciphers such as RC4 or a block
cipher in OFB mode, and SHOULD NOT include a block cipher in ECB mode.
2.2.1 Rationale
The use of a level of indirection in specifying the
KeyEncryptionAlgorithmIdentifier allows alternative wrapping algorithms
to be used in the future. If the KEK algorithm were specified directly
in this field then any use of an alternative wrapping algorithm would
require a change to the PasswordRecipientInfo structure rather than
simply a change to the key encryption algorithm identifier.
The parameter field for this algorithm identifier could be specified to
default to triple-DES, however due to the confusion over NULL vs absent
parameters in algorithm identifiers it's left explicit with no default
value.
2.3.1 Key Wrap
The key wrap algorithm encrypts a CEK with a KEK in a manner which
ensures that every bit of plaintext effects every bit of ciphertext.
This makes it equivalent in function to the package transform [PACKAGE]
without requiring additional mechanisms or resources such as hash
functions or cryptographically strong random numbers. The key wrap
algorithm is performed in two phases, a first phase which formats the
CEK into a form suitable for encryption by the KEK, and a second phase
which wraps the formatted CEK using the KEK.
Key formatting: Create a formatted CEK block consisting of the
following:
1. A one-byte count of the number of bytes in the CEK.
2. A check value containing the bitwise complement of the first three
bytes of the CEK.
3. The CEK.
4. Enough random padding data to make the CEK data block a multiple
of the KEK block length and at least two KEK cipher blocks long
(the fact that 32 bits of count+check value are used means that
even with a 40-bit CEK, the resulting data size will always be at
least two (64-bit) cipher blocks long). The padding data does not
have to be cryptographically strong, although unpredictability
helps. Note that PKCS #5 padding is not used, since the length of
the data is already known.
The formatted CEK block then looks as follows:
CEK byte count || check value || CEK || padding (if required)
Key wrapping:
1. Encrypt the padded key using the KEK.
2. Without resetting the IV (that is, using the last ciphertext block
as the IV), encrypt the encrypted padded key a second time.
The resulting double-encrypted data is the EncryptedKey.
2.3.2 Key Unwrap
Key unwrapping:
1. Using the n-1'th ciphertext block as the IV, decrypt the n'th
ciphertext block.
2. Using the decrypted n'th ciphertext block as the IV, decrypt the
1st ... n-1'th ciphertext blocks. This strips the outer layer of
encryption.
3. Decrypt the inner layer of encryption using the KEK.
Key format verification:
1a.If the CEK byte count is less than the minimum allowed key size
(usually 5 bytes for 40-bit keys) or greater than the wrapped CEK
length or not valid for the CEK algorithm (eg not 16 or 24 bytes
for triple DES), the KEK was invalid.
1b.If the bitwise complement of the key check value doesn't match the
first three bytes of the key, the KEK was invalid.
2.3.3 Example
Given a content-encryption algorithm of Skipjack and a KEK algorithm of
Triple- DES, the wrap steps are as follows:
1. Set the first 4 bytes of the CEK block to the Skipjack key size
(10 bytes) and the bitwise complement of the first three bytes of
the CEK.
2. Append the 80-bit (10-byte) Skipjack CEK and pad the total to 16
bytes (two triple-DES blocks) using 2 bytes of random data.
2. Using the IV given in the KeyEncryptionAlgorithmIdentifer,
encrypted the padded Skipjack key.
3. Without resetting the IV, encrypt the encrypted padded key a second time.
The unwrap steps are as follows:
1. Using the first 8 bytes of the double-encrypted key as the IV,
decrypt the second 8 bytes.
2. Without resetting the IV, decrypt the first 8 bytes.
3. Decrypt the inner layer of encryption using the the IV given in
the KeyEncryptionAlgorithmIdentifer to recover the padded Skipjack
key.
4. If the length byte isn't equal to the Skipjack key size (80 bits
or 10 bytes) or the bitwise complement of the check bytes doesn't
match the first three bytes of the CEK, the KEK was invalid.
2.3.4 Rationale for the Double Wrapping
If many CEK's are encrypted in a standard way with the same KEK and the
KEK has a 64-bit block size then after about 2^32 encryptions there is
a high probability of a collision between different blocks of encrypted
CEK's. If an opponent manages to obtain a CEK, they may be able to
solve for other CEK's. The double-encryption wrapping process, which
makes every bit of ciphertext dependent on every bit of the CEK,
eliminates this collision problem (as well as preventing other
potential problems such as bit-flipping attacks). Since the IV is
applied to the inner layer of encryption, even wrapping the same CEK
with the same KEK will result in a completely different wrapped key
each time.
An additional feature of the double wrapping is that it doens't require
the use of any extra algorithms such as hash algorithms in addition to
the wrapping algorithm itself, allowing it to be implemented in devices
which only support one type of encryption algorithm. A typical example
of such a device is a crypto token such as a smart card which often
only supports a single block cipher and a single public-key algorithm,
making it impossible to wrap keys if the use of an additional algorithm
were required.
3. Test Vectors
This section contains two sets of test vectors, a very basic set for
DES which can be used to verify correctness and which uses an algorithm
which is freely exportable from the US, and a stress-test version which
uses very long passphrase and key sizes and a mixture of algorithms
which can be used to verify the behaviour in extreme cases.
The basic test contains two subtests, a known-answer test for the key
derivation stage and a full test of the key wrapping. Both tests use a
DES-CBC key derived from the password "password" with salt { 12 34 56
78 78 56 34 12 } using 5 iterations of PBKDF2. In the known answer
test the IV is set to all zeroes (equivalent to using ECB) and used to
encrypt an all-zero data block.
The following values are obtained for the known-answer test:
PKCS #5v2 values:
input 70 61 73 73 77 6f 72 64
passphrase: "password"
input salt: 12 34 56 78 78 56 34 12
iterations: 5
output key: D1 DA A7 86 15 F2 87 E6
known answer: 9B BD 78 FC 11 A3 A9 08
The following values are obtained when wrapping a 64-bit (parity-
adjusted) DES-EBC key:
PKCS #5v2 values:
input 70 61 73 73 77 6f 72 64
passphrase: "password"
input salt: 12 34 56 78 78 56 34 12
iterations: 5
output key: D1 DA A7 86 15 F2 87 E6
CEK formatting phase:
length byte: 08
key check: 73 9D 83
CEK: 8C 62 7C 89 73 23 A2 F8
padding: C4 36 F5 41
complete 08 73 9D 83 8C 62 7C 89 73 23 A2 F8 C4 36 F5 41
CEK block:
Key wrap phase (wrap CEK block using 3DES key):
IV: EF E5 98 EF 21 B3 3D 6D
first encr. 06 A0 43 86 1E 82 88 E4 8B 59 9E B9 76 10 00 D4
pass output:
second encr. B8 1B 25 65 EE 37 3C A6 DE DC A2 6A 17 8B 0C 10
pass output:
ASN.1 encoded PasswordRecipientInfo:
0 A3 68: [3] {
2 02 1: INTEGER 0
5 A0 26: [0] {
7 06 9: OBJECT IDENTIFIER id-PBKDF2 (1 2 840 113549 1 5 12)
18 30 13: SEQUENCE {
20 04 8: OCTET STRING
: 12 34 56 78 78 56 34 12
30 02 1: INTEGER 5
: }
: }
34 30 32: SEQUENCE {
36 06 11: OBJECT IDENTIFIER id-alg-PWRI-KEK (1 2 840 113549 1 9 16 3 9)
33 30 17: SEQUENCE {
35 06 5: OBJECT IDENTIFIER des-CBC (1 3 14 3 2 7)
42 04 8: OCTET STRING
: EF E5 98 EF 21 B3 3D 6D
: }
: }
68 04 16: OCTET STRING
: B8 1B 25 65 EE 37 3C A6 DE DC A2 6A 17 8B 0C 10
: }
The following values are obtained when wrapping a 256-bit key (for
example one for AES or Blowfish) using a triple DES-CBC key derived
from the passphrase "All n-entities must communicate with other n-
entities via n-1 entiteeheehees" with salt { 12 34 56 78 78 56 34 12}
using 500 iterations of PBKDF2.
PKCS #5v2 values:
input 41 6C 6C 20 6E 2D 65 6E 74 69 74 69 65 73 20 6D
passphrase: 75 73 74 20 63 6F 6D 6D 75 6E 69 63 61 74 65 20
77 69 74 68 20 6F 74 68 65 72 20 6E 2d 65 6E 74
69 74 69 65 73 20 76 69 61 20 6E 2D 31 20 65 6E
74 69 74 65 65 68 65 65 68 65 65 73
"All n-entities must communicate with other "
"n-entities via n-1 entiteeheehees"
input
salt: 12 34 56 78 78 56 34 12
iterations: 500
output 6A 89 70 BF 68 C9 2C AE A8 4A 8D F2 85 10 85 86
3DES key: 07 12 63 80 CC 47 AB 2D
CEK formatting phase:
length byte: 20
key check: 73 9C 82
CEK: 8C 63 7D 88 72 23 A2 F9 65 B5 66 EB 01 4B 0F A5
D5 23 00 A3 F7 EA 40 FF FC 57 72 03 C7 1B AF 3B
padding: FA 06 0A 45
complete 20 73 9C 82 8C 63 7D 88 72 23 A2 F9 65 B5 66 EB
CEK block: 01 4B 0F A5 D5 23 00 A3 F7 EA 40 FF FC 57 72 03
C7 1B AF 3B FA 06 0A 45
Key wrap phase (wrap CEK block using 3DES key):
IV: BA F1 CA 79 31 21 3C 4E
first encr. F8 3F 9E 16 78 51 41 10 64 27 65 A9 F5 D8 71 CD
pass output: 27 DB AA 41 E7 BD 80 48 A9 08 20 FF 40 82 A2 80
96 9E 65 27 9E 12 6A EB
second encr. C0 3C 51 4A BD B9 E2 C5 AA C0 38 57 2B 5E 24 55
pass output: 38 76 B3 77 AA FB 82 EC A5 A9 D7 3F 8A B1 43 D9
EC 74 E6 CA D7 DB 26 0C
ASN.1 encoded PasswordRecipientInfo:
0 A3 96: [3] {
2 02 1: INTEGER 0
5 A0 27: [0] {
7 06 9: OBJECT IDENTIFIER id-PBKDF2 (1 2 840 113549 1 5 12)
18 30 14: SEQUENCE {
20 04 8: OCTET STRING
: 12 34 56 78 78 56 34 12
30 02 2: INTEGER 500
: }
: }
34 30 35: SEQUENCE {
36 06 11: OBJECT IDENTIFIER id-alg-PWRI-KEK (1 2 840 113549 1 9 16 3 9)
34 30 20: SEQUENCE {
36 06 8: OBJECT IDENTIFIER des-EDE3-CBC (1 2 840 113549 3 7)
46 04 8: OCTET STRING
: BA F1 CA 79 31 21 3C 4E
: }
: }
71 04 40: OCTET STRING
: C0 3C 51 4A BD B9 E2 C5 AA C0 38 57 2B 5E 24 55
: 38 76 B3 77 AA FB 82 EC A5 A9 D7 3F 8A B1 43 D9
: EC 74 E6 CA D7 DB 26 0C
: }
4. Security Considerations
The security of this recipient information type rests on the security
of the underlying mechanisms employed, for which further information
can be found in RFC 2630 and PKCS5v2. More importantly, however, when
used with a password the security of this information type rests on the
entropy of the user-selected password, which is typically quite low.
Pass phrases (as opposed to simple passwords) are STRONGLY RECOMMENDED,
although it should be recognized that even with pass phrases it will be
difficult to use this recipient information type to derive a KEK with
sufficient entropy to properly protect a 128-bit (or higher) CEK.
5. IANA Considerations
The PasswordRecipientInfo key encryption algorithms are identified by
object identifiers (OIDs). OIDs were assigned from an arc contributed
to the S/MIME Working Group by the RSA Security. Should additional
compression algorithms be introduced, the advocates for such algorithms
are expected to assign the necessary OIDs from their own arcs. No
action by the IANA is necessary for this document or any anticipated
updates.
Acknowledgments
The author would like to thank Jim Schaad, Phil Griffin, and the
members of the S/MIME Working Group for their comments and feedback on
this document.
Author Address
Peter Gutmann
University of Auckland
Private Bag 92019
Auckland, New Zealand
pgut001@cs.auckland.ac.nz
References
ASN1 CCITT Recommendation X.208: Specification of Abstract Syntax
Notation One (ASN.1), 1988.
RFC2119 Key Words for Use in RFCs to Indicate Requirement Levels,
S.Bradner, March 1997.
RFC2630 Cryptographic Message Syntax, R.Housley, June 1999.
RFC2898 PKCS #5: Password-Based Cryptography Specification, Version
2.0, B.Kaliski, September 2000.
PACKAGE All-or-Nothing Encryption and the Package Transform,
R.Rivest, Proceedings of Fast Software Encryption '97, Haifa,
Israel, January 1997.
Appendix A: ASN.1:1988 Module
PasswordRecipientInfo-88
{ iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) modules(0) pwri(17) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
IMPORTS
AlgorithmIdentifier
FROM AuthenticationFramework { joint-iso-itu-t ds(5) module(1)
authenticationFramework(7) 3 }
CMSVersion, EncryptedKey
FROM CryptographicMessageSyntax { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) modules(0) cms(1) };
-- The following PDU is defined in PKCS5 { iso(1) member-body(2)
-- us(840) rsadsi(113549) pkcs(1) pkcs-5(5) modules(16)
-- pkcs5v2-0(1) }, however it can't be imported because because
-- it's specified in 1994/1997 ASN.1. Because of this it's copied
-- here from the source but rephrased as 1988 ASN.1. Further
-- details are given in [RFC 2898].
PBKDF2-params ::= SEQUENCE {
salt OCTET STRING,
iterationCount INTEGER (1..MAX),
keyLength INTEGER (1..MAX) OPTIONAL,
prf AlgorithmIdentifier
DEFAULT { algorithm id-hmacWithSHA1, parameters NULL } }
-- The PRF algorithm is also defined in PKCS5 and can neither be
-- imported nor expressed in 1988 ASN.1, however it is encoded as
-- an AlgorithmIdentifier with the OID:
id-hmacWithSHA1 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) digestAlgorithm(2) 7 }
-- and NULL parameters. Further details are given in [RFC 2898].
-- Implementation note: Because of the inability to precisely
-- specify the PBKDF2 PDU or its parameters in 1988 ASN.1, it is
-- likely that implementors will also encounter alternative
-- interpretations of these parameters, usually using an alternate
-- OID from the IPsec arc which is generally used for HMAC-SHA1:
--
-- hMAC-SHA1 OBJECT IDENTIFIER ::= { iso(1)
-- identified-organization(3) dod(6) internet(1) security(5)
-- mechanisms(5) 8 1 2 }
--
-- with absent (rather than NULL) parameters.
-- The PasswordRecipientInfo
id-alg-PWRI-KEK OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 9 }
PasswordRecipientInfo ::= SEQUENCE {
version CMSVersion, -- Always set to 0
keyDerivationAlgorithm
[0] KeyDerivationAlgorithmIdentifier OPTIONAL,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
encryptedKey EncryptedKey }
KeyDerivationAlgorithmIdentifier ::= AlgorithmIdentifier
KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier
END -- PasswordRecipientInfo-88 --
Appendix B: ASN.1:1997 Module
This appendix contains the same information as Appendix A in a more
recent (and precise) ASN.1 notation, however Appendix A takes
precedence in case of conflict.
PasswordRecipientInfo-97
{ iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) modules(0) pwri(18) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
IMPORTS
id-PBKDF2, PBKDF2-params,
FROM PKCS5 { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
pkcs-5(5) }
CMSVersion, EncryptedKey, des-ede3-cbc, CBCParameter
FROM CryptographicMessageSyntax { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) modules(0) cms(1) };
id-alg-PWRI-KEK OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 9 }
PasswordRecipientInfo ::= SEQUENCE {
version CMSVersion, -- Always set to 0
keyDerivationAlgorithm
[0] KeyDerivationAlgorithmIdentifier OPTIONAL,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
encryptedKey EncryptedKey }
KeyDerivationAlgorithmIdentifier ::=
AlgorithmIdentifier {{ KeyDerivationAlgorithms }}
KeyDerivationAlgorithms ALGORITHM ::= {
{ OID id-PBKDF2 PARMS PBKDF2-params },
...
}
KeyEncryptionAlgorithmIdentifier ::=
AlgorithmIdentifier {{ KeyEncryptionAlgorithms }}
KeyEncryptionAlgorithms ALGORITHM ::= {
{ OID id-alg-PWRI-KEK PARMS
AlgorithmIdentifier {{ PWRIAlgorithms }} },
...
}
-- Algorithm identifiers for algorithms used with the
-- id-alg-PWRI-KEK key wrap algorithm. Currently only 3DES is a
-- MUST, all others are optional
PWRIAlgorithms ALGORITHM ::= {
{ OID des-ede3-cbc PARMS CBCParameter },
...
}
-- Supporting definitions. We could also pull in the
-- AlgorithmIdentifier from an appropriately recent X.500 module (or
-- wherever) but it's just as easy (and more convenient for readers)
-- to provide a definition here
AlgorithmIdentifier { ALGORITHM:IOSet } ::= SEQUENCE {
algorithm ALGORITHM.&id({IOSet}),
parameters ALGORITHM.&Type({IOSet}{@algorithm}) OPTIONAL
}
ALGORITHM ::= CLASS {
&id OBJECT IDENTIFIER UNIQUE,
&Type OPTIONAL
}
WITH SYNTAX { OID &id [PARMS &Type] }
END -- PasswordRecipientInfo-97 --
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